I want to prevent a cell from being recalculated (Excel 2000).

For Example:
December Widget price is £1.00 (Item/Price held in a Range)
Sold 500 - Income = £500
January updated Item/Price range, Widget price now £1.25
Sold 200 - income = £250
BUT...
December income now INCORRECTLY shows income = £625.
How can I stop the December values from being updated (Recalculated) once
they have been calculated?

Thanks for any help.